In order to abate emission of carbon dioxide gas from automobiles, a reduction in weight of automobile vehicle bodies has been promoted by using high-strength steel sheets. Further, in order also to secure the safety of a passenger, a high-strength steel sheet has been increasingly used for an automobile vehicle body in addition to a soft steel sheet. In order to further promote the reduction in weight of automobile vehicle bodies from now on, it is necessary to increase the level of usage strength of a high-strength steel sheet more than conventionally. However, when a high-strength steel sheet is used for an outer panel part, cutting, blanking, and the like are often applied, and further when a high-strength steel sheet is used for an underbody part, working methods accompanied by shearing such as punching are often applied, resulting in that a steel sheet having excellent precision punchability has been required. Further, workings such as burring have also been increasingly performed after shearing, so that stretch flangeability is also an important property related to working. However, when a steel sheet is increased in strength in general, punching accuracy decreases and stretch flangeability also decreases.
With regard to the precision punchability, as is in Patent Documents 1 and 2, there is disclosed that punching is performed in a soft state and achievement of high strength is attained by heat treatment and carburization, but a manufacturing process is prolonged to thus cause an increase in cost. On the other hand, as is in Patent Document 3, there is also disclosed a method of improving precision punchability by spheroidizing cementite by annealing, but achievement of stretch flangeability important for working of automobile vehicle bodies and the like and the precision punchability is not considered at all.
With regard to the stretch flangeability to achievement of high strength, a steel sheet metal structure control method to improve local elongation is also disclosed, and Non-Patent Document 1 discloses that controlling inclusions, making structures uniform, and further decreasing difference in hardness between structures are effective for bendability and stretch flangeability. Further, Non-Patent Document 2 discloses a method in which a finishing temperature of hot rolling, a reduction ratio and a temperature range of finish rolling are controlled, recrystallization of austenite is promoted, development of a rolled texture is suppressed, and crystal orientations are randomized, to thereby improve strength, ductility, and stretch flangeability.
From Non-Patent Documents 1 and 2, it is conceivable that the metal structure and rolled texture are made uniform, thereby making it possible to improve the stretch flangeability, but the achievement of the precision punchability and the stretch flangeability is not considered at all.